The recent mainstream of the covering material for thread-wound and two-piece golf balls is an ionomer resin characterized by high repulsion and cut resistance. Since golf players have a continuous demand for higher flying performance and durability, it is desired to develop a covering material of higher quality.
The ionomer resin covering material generally contains a pigment for increasing the whiteness of the material, for example, titanium dioxide and zinc oxide, and a dispersant for the pigment, for example, fatty acid metal salts such as magnesium stearate. The covering material was applied to thread-wound cores or solid cores by compression molding of half cups or injection molding. However, either of the molding methods often failed to obtain a fully impact-resistant cover layer because the ionomer resin in the covering material has a high degree of orientation. It was thus difficult to manufacture golf balls having sufficient durability to fully meet the demand of golf players. Further, the fatty acid metal salt dispersants such as magnesium stearate tend to gasify upon injection of the covering material, undesirably sticking to the mold surface. Such deposits can also adhere to the cover surface to adversely affect its receptivity to a lacquer coating.
In the prior art, it was proposed to add linear low-density polyethylene and medium-density polyethylene to ionomer resin based covering materials in order to improve their impact resistance (see Japanese Patent Publication Nos. 58855/1988 and 58856/1988). These covering materials, however, suffer from variations in physical properties due to the poor dispersion of such polyethylene.